1. Field of the Invention
The present invention relates to a voltage controlled oscillator, particularly, to a voltage controlled oscillator equipped with a thin film BAW resonator that is utilized in a mobile wireless communication technology such as a portable telephone.
2. Description of the Related Art
The conventional voltage controlled oscillator comprises a tank circuit including an inductor formed of a coil or a strip line and a variable capacitor element using a varactor diode having the capacitance value changed by the voltage. The conventional voltage controlled oscillator is advantageous in that the frequency can be varied over a wide range that is not smaller than 10%, but is defective in that the inductor included in the voltage controlled oscillator is large, and that the phase noise is large because the Q value (mechanically quality coefficient) of the inductor or the varactor is small.
Recently, a voltage controlled oscillator utilizing a thin film BAW (Bulk Acoustic Wave) resonator, i.e., Film Bulk Acoustic Wave Resonator (FBAR), attracts attentions. The thin film BAW resonator has an element structure prepared by successively forming a lower electrode, a piezoelectric film, and an upper electrode one upon the other on a substrate with a gap interposed between the lower electrode and the substrate.
A voltage controlled oscillator comprising a tank circuit formed of the thin film BAW resonator and the varactor diode is disclosed in, for example, “A 2 GHz voltage tunable FBAR oscillator” by A.P.S. Khanna, E. Gane and T. Chong, which is reported in “2003 IEEE MTT-S Digest, pp. 717-20, 2003”. The voltage controlled oscillator disclosed in this literature has a frequency variable width that is determined by the electromechanical coupling constant (k2) of the thin film BAW resonator and the capacitance change ratio of the varactor diode. When it comes to a combination of, for example, a thin film BAW resonator using a piezoelectric film formed of AlN and a varactor diode, the constant k2 is about 6% and the capacitance change ratio, i.e., the ratio of the maximum capacitance to the minimum capacitance, is at most about 4. As a result, the range within which the frequency can be changed is small, i.e., about 2 to 3%. It follows that the particular combination noted above gives rise to the problem that the variable range of the frequency is excessively small when the combination is used in the voltage controlled oscillator used in many mobile wireless systems.
It should also be noted that that the Q value of the varactor diode is markedly smaller than the Q value of the thin film BAW resonator, with the result that the Q value of the oscillator is lowered so as to give rise to the problem that the phase noise is increased.
As described above, the voltage controlled oscillator using a thin film BAW resonator and a varactor diode is defective in that the frequency variable range is small and that the phase noise is large.